This invention relates in general to Schottky barrier diodes that are characterized by low barrier height (in the region 0.15 volt) suitable for use without dc bias as a detector at microwave frequencies and more particularly to a metal-silicide-silicon diode.
Schottky barrier diodes are particularly attractive for use in microwave circuit assemblies made with stripline, microstrip and similar kinds of transmission lines using conductors formed as strips on a dielectric substrate, because they may be used in the unpackaged chip form and they are bondable to the conductors used in such lines. However, additional dc bias circuitry that is presently required to bias Schottky barrier diodes to operate them as detectors in stripline, microstrip and like transmission line assemblies is expensive, complex, and affects adversely the overall performance of the diodes as detectors at microwave frequencies in such assemblies. The invention accordingly relates also to zero-bias bondable detector diodes, that can be used in stripline, microstrip and like transmission line assemblies without dc bias circuitry, and therefore without introducing any requirement for dc offset compensating circuitry.
Point contact, Schottky-barrier, and back diodes are used as mixers and detectors from UHF to millimeter frequencies. A discussion of properties of these devices appears in an article entitles "Microwave Mixer and Detector Diodes" by the present inventor and W. J. Moroney, Proc. IEEE, Vol. 59, No. 8, August, 1971, pages 1182-1190. At the present time, to my knowledge, there are no bondable zero bias detectors available.
Metal semiconductor contacts can be ohmic or rectifying. Those made on heavily doped semiconductors generally are ohmic, and such contacts made on lightly doped semiconductor materials exhibit rectifying behavior with an energy barrier existing between the metal and the semiconductor. Point contact and Schottky barrier diodes belong to the second (rectifying behavior) category.
Point contacts diodes are made by a pressure contact between a metal and a semiconductor body, and optimum whisker pressure is needed to obtain the rectifying barrier. Generally, point contact diodes are attractive for use at microwave frequencies because they have extremely low energy barrier between the metal and the semiconductor, and they can be used without dc bias as rectifiers (i.e.: they can be operated as zero bias detectors) at microwave frequencies. They are generally available only in packages which are not suitable for bonding to the conductors of stripline or microstrip transmission lines.
A Schottky barrier diode is formed by plating, evaporating or sputtering a variety of metals and alloys on n- or p-type semiconductor materials. All types of Schottky barrier diodes that are presently available exhibit energy barrier heights from about 0.24 volt to about 0.90 volt, and they all require an additional dc bias to operate them as detectors at microwave frequencies.
The energy barrier height of the Schottky barrier diode is governed approximately by the difference in the work function of the metal and that of the semiconductor (see U.S. Pat. No. 3,599,322; column 2, lines 19-22). In practice it is found that a prediction of rectifying properties based on the work functions of pure metals is not accurate. The actual barrier junction on silicon is in most cases a metal-silicide junction, and impurities are involved on both sides. The barrier height or forward voltage at which current starts (turn-on voltage) is therefore influenced by diffusion during processing. Thus, while choice of metals for low-barrier diodes can be helped by the work function theory, actual turn-on voltages are not predictable.
A relatively early article by Kahng and Lepselter entitled "Planar Epitaxial Silicon Schottky Barrier Diodes" (Bell System Technical Journal, Vol. 44, Sept. 1965, pages 1525-1528) describes formation of metal silicides by evaporation of the metal over the exposed silicon at temperatures of 300.degree.C to 700.degree.C. The silicide rectifying junctions have lower barrier height than pure metal junctions (0.87 volt for Pt-silicide-Si vs. 1.00 for Pt-Si, for example). It is noted, however, that the barrier height is usually increased by higher temperatures until it approaches the pure metal value. In the U.S. Pat. No. 3,599,056 of Lepselter and MacRae a metal-silicide is formed on n-type silicon at a temperature in excess of 400.degree.C, usually of the order of 700.degree.C to promote formation of the silicide layer (column 2, lines 8-25). Silicide forming metals suggested are Ni, Ti, Zr, Hf, and the six platinum group metals. The same teaching appears in U.S. Pat. Nos. 3,604,986 and 3,616,380 of Lepselter et al. Kahng and Lepselter's U.S. Pat. No. 3,290,127 describes a Schottky barrier diode using palladium silicide as the active contact layer producing the surface barrier with the semiconductor material. The silicide is formed in a palladium and silicon solid state reaction at 400.degree.C or higher. Lepselter's U.S. Pat. No. 3,287,612 describes a use of platinum silicide formed in a solid phase reaction at a temperature between 500.degree. and 600.degree.C, to make an ohmic contact to p-type silicon.
D'Heurle, Esaki and Seki in U.S. Pat. No. 3,451,912 proposed to form a metal-silicide as the metal-semiconductor junction, using sputtered metal ions deposited in intimate contact with the surface of the semiconductor body (column 2, lines 47-51; column 3, line 4). The junction thus formed is stated to exhibit rectifying properties when --appropriately biased (column 3, lines 49-51). A threshold voltage in the order of 0.35 volt is stated for a molybdenum-silicon barrier diode (column 7, lines 40-42).
Crowell and Sze in U.S. Pat. No. 3,349,297 mention converting a portion of a platinum layer on silicon to platinum silicide to improve adherence of the layer. This patent suggests several metal-silicon combinations for making a rectifying surface barrier contact, but the significant teaching of the patent is to use two metals having relatively different barrier heights, one to form a surface barrier electrode, and the other as a guard ring to control the area and shape of the first, as well as to provide a means for making electrical contact to it (column 2, lines 36-40; column 4, lines 63-65).
Dhaka's U.S. Pat. No. 3,506,893 describes (column 5, lines 4-14) sintering platinum on silicon at about 550.degree.C for 20 minutes to form an alloy of platinum and silicon (platinum silicide), for use as an ohmic contact or a Schottky-barrier diode (column 5, lines 31-36). This diode is a biased switching diode, operated with a voltage of (+)0.4 volts on its anode (column 5, lines 42-45), and a greater voltage barrier appears to be desired (column 5, lines 73-75).
Saxena's U.S. Pat. No. 3,700,979 provides a Schottky barrier diode having a high barrier height (approximately 0.90 volt) by annealing a film of hafnium on p-type silicon at a temperature between 450.degree.C and 550.degree.C. Saxena's U.S. Pat. No. 3,599,323 describes a hot carrier diode using a chromium-silicon barrier, which is stated to have low-turn-on voltage, about 0.27 volt, made by vapor-depositing chromium on p-type silicon. No temperature range is specified for the process.